The present invention relates to leakage current cancellation in string digital-to-analog converters (DACs).
String DACs convert a digital word into a corresponding analog signal. A dual string DAC, which is a string DAC type, includes two impedance strings such as two series connected resistor strings and two switch networks. The outputs of the first resistor string, which typically is used for converting the most significant bits (MSBs) of the digital word, is coupled to the input of the second resistor string, which typically is used for converting the least significant bits (LSBs) of the digital word. Thus, for medium-to-high resolution converters, the voltages at selected first resistor terminals are often close in value. The selected resistor in first resistor string, first switching network, and second resistor string may be operating at similar common-mode.
An error source in string DACs operating at a given common-mode is leakage current, which can corrupt the conversion. Leakage current can be present at the outputs of the switches in the two switch networks and can refer to the undesired current that “leaks” in to or from switch terminals. The impact of such leakage current becomes more substantial in low power and/or high temperature applications. In low power applications, small amounts of leakage current can lead to significant errors because of the low voltages used in the conversion. In high temperature applications, leakage current increases proportionally to increasing temperature (e.g., leakage current can substantially double every 10 degrees Celsius).
Therefore, the inventor recognized a need in the art for efficient leakage current cancellation.